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bh=Df5LVFvtrQu6/8aH2DWTvdQzTx5gKr69K5KXG9KebXs=; b=V9BBvmaWXIH68pK2owGINcSSQjiqe3ZfShOGq2s7KUrr/04qLfRDlOzmHD+0okknE1Dzgd O0v94icXEEMlTwIJEr6NGVgaFYCxE57hG8NxWz6/5lUSSBs1JWqTLWw/qRO+coyCx3mS+K ZNpwSzdJoZfPxW1cNafO3p9p12CeHTU= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-590-iZPaT-zyOGGYRRtLopoVdQ-1; Thu, 19 Jan 2023 10:29:29 -0500 X-MC-Unique: iZPaT-zyOGGYRRtLopoVdQ-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id B16A91C189A9; Thu, 19 Jan 2023 15:29:28 +0000 (UTC) Received: from warthog.procyon.org.uk.com (unknown [10.33.36.23]) by smtp.corp.redhat.com (Postfix) with ESMTP id 977FB2166B2C; Thu, 19 Jan 2023 15:29:27 +0000 (UTC) From: David Howells To: dhowells@kvack.org Cc: David Howells , Al Viro , Christoph Hellwig , John Hubbard , Matthew Wilcox , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH] iov_iter: Add a function to extract a page list from an iterator Date: Thu, 19 Jan 2023 15:29:26 +0000 Message-Id: <20230119152926.2899954-1-dhowells@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 1FBD9100017 X-Stat-Signature: mxphx7tuomf895tw4d1ni6qijr73r6p8 X-HE-Tag: 1674142172-510929 X-HE-Meta: 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 J1g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Add a function, iov_iter_extract_pages(), to extract a list of pages from an iterator. The pages may be returned with a reference added or a pin added or neither, depending on the type of iterator and the direction of transfer. The caller must pass FOLL_READ_FROM_MEM or FOLL_WRITE_TO_MEM as part of gup_flags to indicate how the iterator contents are to be used. Add a second function, iov_iter_extract_mode(), to determine how the cleanup should be done. There are three cases: (1) Transfer *into* an ITER_IOVEC or ITER_UBUF iterator. Extracted pages will have pins obtained on them (but not references) so that fork() doesn't CoW the pages incorrectly whilst the I/O is in progress. iov_iter_extract_mode() will return FOLL_PIN for this case. The caller should use something like unpin_user_page() to dispose of the page. (2) Transfer is *out of* an ITER_IOVEC or ITER_UBUF iterator. Extracted pages will have references obtained on them, but not pins. iov_iter_extract_mode() will return FOLL_GET. The caller should use something like put_page() for page disposal. (3) Any other sort of iterator. No refs or pins are obtained on the page, the assumption is made that the caller will manage page retention. ITER_ALLOW_P2PDMA is not permitted. iov_iter_extract_mode() will return 0. The pages don't need additional disposal. Signed-off-by: David Howells cc: Al Viro cc: Christoph Hellwig cc: John Hubbard cc: Matthew Wilcox cc: linux-fsdevel@vger.kernel.org cc: linux-mm@kvack.org --- Notes: ver #7) - Switch to passing in iter-specific flags rather than FOLL_* flags. - Use READ_FROM_ITER/WRITE_TO_ITER to indicate direction. - Use ITER_ALLOW_P2PDMA to request FOLL_PCI_P2PDMA. - Disallow use of ITER_ALLOW_P2PDMA with non-user-backed iter. ver #6) - Add back the function to indicate the cleanup mode. - Drop the cleanup_mode return arg to iov_iter_extract_pages(). - Pass FOLL_SOURCE/DEST_BUF in gup_flags. Check this against the iter data_source. ver #4) - Use ITER_SOURCE/DEST instead of WRITE/READ. - Allow additional FOLL_* flags, such as FOLL_PCI_P2PDMA to be passed in. ver #3) - Switch to using EXPORT_SYMBOL_GPL to prevent indirect 3rd-party access to get/pin_user_pages_fast()[1]. Link: https://lore.kernel.org/r/166920903885.1461876.692029808682876184.stgit@warthog.procyon.org.uk/ # v2 Link: https://lore.kernel.org/r/166997421646.9475.14837976344157464997.stgit@warthog.procyon.org.uk/ # v3 Link: https://lore.kernel.org/r/167305163883.1521586.10777155475378874823.stgit@warthog.procyon.org.uk/ # v4 Link: https://lore.kernel.org/r/167344728530.2425628.9613910866466387722.stgit@warthog.procyon.org.uk/ # v5 include/linux/uio.h | 28 ++++ lib/iov_iter.c | 370 ++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 398 insertions(+) diff --git a/include/linux/uio.h b/include/linux/uio.h index 4348d37c008b..bac0852de820 100644 --- a/include/linux/uio.h +++ b/include/linux/uio.h @@ -356,4 +356,32 @@ static inline void iov_iter_ubuf(struct iov_iter *i, unsigned int direction, #define WRITE_TO_ITER 0x02 /* We will be writing data to the iterator */ #define ITER_ALLOW_P2PDMA 0x04 /* Allow P2PDMA on the extracted pages */ +ssize_t iov_iter_extract_pages(struct iov_iter *i, struct page ***pages, + size_t maxsize, unsigned int maxpages, + unsigned int extract_flags, size_t *offset0); + +/** + * iov_iter_extract_mode - Indicate how pages from the iterator will be retained + * @iter: The iterator + * @gup_flags: How the iterator is to be used (FOLL_WRITE_TO/READ_FROM_MEM) + * + * Examine the iterator and the gup_flags and indicate by returning FOLL_PIN, + * FOLL_GET or 0 as to how, if at all, pages extracted from the iterator will + * be retained by the extraction function. + * + * FOLL_GET indicates that the pages will have a reference taken on them that + * the caller must put. This can be done for DMA/async DIO write from a page. + * + * FOLL_PIN indicates that the pages will have a pin placed in them that the + * caller must unpin. This is must be done for DMA/async DIO read to a page to + * avoid CoW problems in fork. + * + * 0 indicates that no measures are taken and that it's up to the caller to + * retain the pages. + */ +#define iov_iter_extract_mode(iter, gup_flags) \ + (user_backed_iter(iter) ? \ + (gup_flags & FOLL_IO_DIR_MASK) == FOLL_READ_FROM_MEM ? \ + FOLL_GET : FOLL_PIN : 0) + #endif diff --git a/lib/iov_iter.c b/lib/iov_iter.c index 77b2192cda68..9e6f6a6257a8 100644 --- a/lib/iov_iter.c +++ b/lib/iov_iter.c @@ -1915,3 +1915,373 @@ void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state) i->iov -= state->nr_segs - i->nr_segs; i->nr_segs = state->nr_segs; } + +/* + * Extract a list of contiguous pages from an ITER_PIPE iterator. This does + * not get references of its own on the pages, nor does it get a pin on them. + * If there's a partial page, it adds that first and will then allocate and add + * pages into the pipe to make up the buffer space to the amount required. + * + * The caller must hold the pipe locked and only transferring into a pipe is + * supported. + */ +static ssize_t iov_iter_extract_pipe_pages(struct iov_iter *i, + struct page ***pages, size_t maxsize, + unsigned int maxpages, + unsigned int gup_flags, + size_t *offset0) +{ + unsigned int nr, offset, chunk, j; + struct page **p; + size_t left; + + if (!sanity(i)) + return -EFAULT; + + offset = pipe_npages(i, &nr); + if (!nr) + return -EFAULT; + *offset0 = offset; + + maxpages = min_t(size_t, nr, maxpages); + maxpages = want_pages_array(pages, maxsize, offset, maxpages); + if (!maxpages) + return -ENOMEM; + p = *pages; + + left = maxsize; + for (j = 0; j < maxpages; j++) { + struct page *page = append_pipe(i, left, &offset); + if (!page) + break; + chunk = min_t(size_t, left, PAGE_SIZE - offset); + left -= chunk; + *p++ = page; + } + if (!j) + return -EFAULT; + return maxsize - left; +} + +/* + * Extract a list of contiguous pages from an ITER_XARRAY iterator. This does not + * get references on the pages, nor does it get a pin on them. + */ +static ssize_t iov_iter_extract_xarray_pages(struct iov_iter *i, + struct page ***pages, size_t maxsize, + unsigned int maxpages, + unsigned int gup_flags, + size_t *offset0) +{ + struct page *page, **p; + unsigned int nr = 0, offset; + loff_t pos = i->xarray_start + i->iov_offset; + pgoff_t index = pos >> PAGE_SHIFT; + XA_STATE(xas, i->xarray, index); + + offset = pos & ~PAGE_MASK; + *offset0 = offset; + + maxpages = want_pages_array(pages, maxsize, offset, maxpages); + if (!maxpages) + return -ENOMEM; + p = *pages; + + rcu_read_lock(); + for (page = xas_load(&xas); page; page = xas_next(&xas)) { + if (xas_retry(&xas, page)) + continue; + + /* Has the page moved or been split? */ + if (unlikely(page != xas_reload(&xas))) { + xas_reset(&xas); + continue; + } + + p[nr++] = find_subpage(page, xas.xa_index); + if (nr == maxpages) + break; + } + rcu_read_unlock(); + + maxsize = min_t(size_t, nr * PAGE_SIZE - offset, maxsize); + i->iov_offset += maxsize; + i->count -= maxsize; + return maxsize; +} + +/* + * Extract a list of contiguous pages from an ITER_BVEC iterator. This does + * not get references on the pages, nor does it get a pin on them. + */ +static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i, + struct page ***pages, size_t maxsize, + unsigned int maxpages, + unsigned int gup_flags, + size_t *offset0) +{ + struct page **p, *page; + size_t skip = i->iov_offset, offset; + int k; + + maxsize = min(maxsize, i->bvec->bv_len - skip); + skip += i->bvec->bv_offset; + page = i->bvec->bv_page + skip / PAGE_SIZE; + offset = skip % PAGE_SIZE; + *offset0 = offset; + + maxpages = want_pages_array(pages, maxsize, offset, maxpages); + if (!maxpages) + return -ENOMEM; + p = *pages; + for (k = 0; k < maxpages; k++) + p[k] = page + k; + + maxsize = min_t(size_t, maxsize, maxpages * PAGE_SIZE - offset); + i->count -= maxsize; + i->iov_offset += maxsize; + if (i->iov_offset == i->bvec->bv_len) { + i->iov_offset = 0; + i->bvec++; + i->nr_segs--; + } + return maxsize; +} + +/* + * Get the first segment from an ITER_UBUF or ITER_IOVEC iterator. The + * iterator must not be empty. + */ +static unsigned long iov_iter_extract_first_user_segment(const struct iov_iter *i, + size_t *size) +{ + size_t skip; + long k; + + if (iter_is_ubuf(i)) + return (unsigned long)i->ubuf + i->iov_offset; + + for (k = 0, skip = i->iov_offset; k < i->nr_segs; k++, skip = 0) { + size_t len = i->iov[k].iov_len - skip; + + if (unlikely(!len)) + continue; + if (*size > len) + *size = len; + return (unsigned long)i->iov[k].iov_base + skip; + } + BUG(); // if it had been empty, we wouldn't get called +} + +/* + * Extract a list of contiguous pages from a user iterator and get references + * on them. This should only be used iff the iterator is user-backed + * (IOBUF/UBUF) and data is being transferred out of the buffer described by + * the iterator (ie. this is the source). + * + * The pages are returned with incremented refcounts that the caller must undo + * once the transfer is complete, but no additional pins are obtained. + * + * This is only safe to be used where background IO/DMA is not going to be + * modifying the buffer, and so won't cause a problem with CoW on fork. + */ +static ssize_t iov_iter_extract_user_pages_and_get(struct iov_iter *i, + struct page ***pages, + size_t maxsize, + unsigned int maxpages, + unsigned int gup_flags, + size_t *offset0) +{ + unsigned long addr; + size_t offset; + int res; + + if (WARN_ON_ONCE(i->data_source != ITER_SOURCE)) + return -EFAULT; + + gup_flags |= FOLL_GET; + if (i->nofault) + gup_flags |= FOLL_NOFAULT; + + addr = iov_iter_extract_first_user_segment(i, &maxsize); + *offset0 = offset = addr % PAGE_SIZE; + addr &= PAGE_MASK; + maxpages = want_pages_array(pages, maxsize, offset, maxpages); + if (!maxpages) + return -ENOMEM; + res = get_user_pages_fast(addr, maxpages, gup_flags, *pages); + if (unlikely(res <= 0)) + return res; + maxsize = min_t(size_t, maxsize, res * PAGE_SIZE - offset); + iov_iter_advance(i, maxsize); + return maxsize; +} + +/* + * Extract a list of contiguous pages from a user iterator and get a pin on + * each of them. This should only be used iff the iterator is user-backed + * (IOBUF/UBUF) and data is being transferred into the buffer described by the + * iterator (ie. this is the destination). + * + * It does not get refs on the pages, but the pages must be unpinned by the + * caller once the transfer is complete. + * + * This is safe to be used where background IO/DMA *is* going to be modifying + * the buffer; using a pin rather than a ref makes sure that CoW happens + * correctly in the parent during fork. + */ +static ssize_t iov_iter_extract_user_pages_and_pin(struct iov_iter *i, + struct page ***pages, + size_t maxsize, + unsigned int maxpages, + unsigned int gup_flags, + size_t *offset0) +{ + unsigned long addr; + size_t offset; + int res; + + if (WARN_ON_ONCE(i->data_source != ITER_DEST)) + return -EFAULT; + + gup_flags |= FOLL_PIN | FOLL_WRITE; + if (i->nofault) + gup_flags |= FOLL_NOFAULT; + + addr = first_iovec_segment(i, &maxsize); + *offset0 = offset = addr % PAGE_SIZE; + addr &= PAGE_MASK; + maxpages = want_pages_array(pages, maxsize, offset, maxpages); + if (!maxpages) + return -ENOMEM; + res = pin_user_pages_fast(addr, maxpages, gup_flags, *pages); + if (unlikely(res <= 0)) + return res; + maxsize = min_t(size_t, maxsize, res * PAGE_SIZE - offset); + iov_iter_advance(i, maxsize); + return maxsize; +} + +static ssize_t iov_iter_extract_user_pages(struct iov_iter *i, + struct page ***pages, size_t maxsize, + unsigned int maxpages, + unsigned int gup_flags, + size_t *offset0) +{ + if (iov_iter_extract_mode(i, gup_flags) == FOLL_GET) + return iov_iter_extract_user_pages_and_get(i, pages, maxsize, + maxpages, gup_flags, + offset0); + else + return iov_iter_extract_user_pages_and_pin(i, pages, maxsize, + maxpages, gup_flags, + offset0); +} + +/** + * iov_iter_extract_pages - Extract a list of contiguous pages from an iterator + * @i: The iterator to extract from + * @pages: Where to return the list of pages + * @maxsize: The maximum amount of iterator to extract + * @maxpages: The maximum size of the list of pages + * @gup_flags: Direction indicator and additional flags + * @offset0: Where to return the starting offset into (*@pages)[0] + * + * Extract a list of contiguous pages from the current point of the iterator, + * advancing the iterator. The maximum number of pages and the maximum amount + * of page contents can be set. + * + * If *@pages is NULL, a page list will be allocated to the required size and + * *@pages will be set to its base. If *@pages is not NULL, it will be assumed + * that the caller allocated a page list at least @maxpages in size and this + * will be filled in. + * + * @gup_flags can be set to either FOLL_READ_FROM_MEM or FOLL_WRITE_TO_MEM, + * indicating how the buffer is to be used, and can have FOLL_PCI_P2PDMA OR'd + * with that. + * + * The iov_iter_extract_mode() function can be used to query how cleanup should + * be performed. + * + * Extra refs or pins on the pages may be obtained as follows: + * + * (*) If the iterator is user-backed (ITER_IOVEC/ITER_UBUF) and data is to be + * transferred /OUT OF/ the buffer (@gup_flags |= FOLL_READ_FROM_MEM), + * refs will be taken on the pages, but pins will not be added. This can + * be used for DMA from a page; it cannot be used for DMA to a page, as it + * may cause page-COW problems in fork. iov_iter_extract_mode() will + * return FOLL_GET. + * + * (*) If the iterator is user-backed (ITER_IOVEC/ITER_UBUF) and data is to be + * transferred /INTO/ the described buffer (@gup_flags |= + * FOLL_WRITE_TO_MEM), pins will be added to the pages, but refs will not + * be taken. This must be used for DMA to a page. + * iov_iter_extract_mode() will return FOLL_PIN. + * + * (*) If the iterator is ITER_PIPE, this must describe a destination for the + * data. Additional pages may be allocated and added to the pipe (which + * will hold the refs), but neither refs nor pins will be obtained for the + * caller. The caller must hold the pipe lock. iov_iter_extract_mode() + * will return 0. + * + * (*) If the iterator is ITER_BVEC or ITER_XARRAY, the pages are merely + * listed; no extra refs or pins are obtained. iov_iter_extract_mode() + * will return 0. + * + * Note also: + * + * (*) Use with ITER_KVEC is not supported as that may refer to memory that + * doesn't have associated page structs. + * + * (*) Use with ITER_DISCARD is not supported as that has no content. + * + * On success, the function sets *@pages to the new pagelist, if allocated, and + * sets *offset0 to the offset into the first page.. + * + * It may also return -ENOMEM and -EFAULT. + */ +ssize_t iov_iter_extract_pages(struct iov_iter *i, + struct page ***pages, + size_t maxsize, + unsigned int maxpages, + unsigned int gup_flags, + size_t *offset0) +{ + + switch (gup_flags & FOLL_IO_DIR_MASK) { + case FOLL_READ_FROM_MEM: + if (WARN_ON_ONCE(i->data_source == ITER_DEST)) + return -EIO; + break; + case FOLL_WRITE_TO_MEM: + if (WARN_ON_ONCE(i->data_source == ITER_SOURCE)) + return -EIO; + break; + default: + WARN_ON_ONCE(1); + return -EIO; + } + + maxsize = min_t(size_t, min_t(size_t, maxsize, i->count), MAX_RW_COUNT); + if (!maxsize) + return 0; + + if (likely(user_backed_iter(i))) + return iov_iter_extract_user_pages(i, pages, maxsize, + maxpages, gup_flags, + offset0); + if (iov_iter_is_bvec(i)) + return iov_iter_extract_bvec_pages(i, pages, maxsize, + maxpages, gup_flags, + offset0); + if (iov_iter_is_pipe(i)) + return iov_iter_extract_pipe_pages(i, pages, maxsize, + maxpages, gup_flags, + offset0); + if (iov_iter_is_xarray(i)) + return iov_iter_extract_xarray_pages(i, pages, maxsize, + maxpages, gup_flags, + offset0); + return -EFAULT; +} +EXPORT_SYMBOL_GPL(iov_iter_extract_pages);